In the electrostatic coating of workpieces, for example vehicle bodies, it is known, and common practice, to pass the workpieces in series through spray-booths in which rotary sprayers, connected individually or in groups to high-voltage generators, are installed. In conventional installations, the coating material was raised to the high-voltage potential directly in or on the atomizer. However, in order that the presently preferred coating materials having higher electrical conductivity may be used, like the so-called water-enamels, it is better to ground the entire enamel feed-system and the atomizing bell, or the like, and to charge the mechanically atomized paint particles by means of the external electrodes surrounding the atomizing bell. Reference of such may be had in German OS 34 290 075 and 36 09 240. Charging is effected by ion-capture occurring in the corona-discharge through the electrode tips.
When an installation of this kind was in operation, it was hitherto customary to stabilize the voltage from the high-voltage generator, applied to the spraying devices, at constant values which, in order to take account of the relative operating conditions, could be adjusted in steps, for example over the range between 60 and 100 kV. If the voltage is held to a constant value, problems arise, especially in connection with the previously mentioned external charging of the coating material in the corona-discharge, since, under certain circumstances, considerable fluctuations in the corona-current could occur while the unit was in operation. The corona-current is distinctly higher than the operating current normally used during conventional contact-charging of the coating material on the atomizer. The corona-current is not only dependent upon the voltage potential at the electrode, but also upon various environmental conditions such as atmospheric humidity and contamination about the electrode area. For example, in a typical coating installation of the type in question, the corona-current may fluctate between 100 and 300 uA when atmospheric humidity varies between 30 and 90%. However, both unduly high and unduly low corona-currents must be avoided. In the case of unduly high corona-currents, adequate ionization fails to occur, resulting in unsatisfactory coating efficiency (the coating efficiency being the ratio between material sprayed and material reaching the workpiece). In the case of unduly low corona-current, there is a danger of overloading the paint mist with space-charging effects which, according to experience, may almost completely suppress the corona-current and ionization. In both cases, as a result of inadequate charging of the paint-particles, there may be rapid contamination of the electrodes, of their holders and of other parts of the atomizer by the material sprayed. Additional problems arise as a result of the pronounced dependency of the corona-current upon voltage fluctuations which result in substantially greater current changes when compared with the relatively minor current changes resulting from voltage fluctuations in the conventional contact charging methods utilizing a lower operating current. Such current-changes are undesirable in practice.
It is known from German OS 34 45 946, in electrostatically coating large workpieces, such as vehicle bodies, to switch the installation off automatically, in order to avoid a voltage breakthrough between the workpiece and the coating device, as soon as the operating current reaches a threshold value which is predetermined as a function of the operating voltage which is adjustable within a certain range. In such an operation, all current threshold values corresponding with their operation voltage-values are stored jointly, more particularly in a microprocessor, and are automatically selected, when the unit is in operation, in accordance with the voltages set. Initially, a warning signal only may be produced when the current, which is measured constantly while the unit is in operation, exceeds an intermediate threshold value between the normal value and the switch-off threshold value.
In the case of a method known from German OS 24 51 818 for the electrostatic coating of workpieces moving at variable distances past spray-discs carrying high voltages, the voltage is held constant until a specific distance is not reached and an adjustable current maximum is reached. Thereafter, for the purpose of limiting the field-strength between the spray-disc and the workpiece, the current is temporarily held constant until the high voltage applied is finally switched off when an adjustable minimal distance is not reached. Apart from the deficiency with this method that the coating material is not at ground potential during spraying and it is not charged in the corona-discharge, all that this method provides is a limit to a maximal current-value which need not be reached during normal operation, depending upon the spacing. The previously mentioned contamination about the electrode may therefore occur, especially during changes in atmospheric humidity and other environmental conditions.